The Thorpe-Ingold effect or gem-dimethyl effect, or angle compression is an effect observed in organic chemistry where increasing the size of two substituents on a tetrahedral center leads to enhanced reactions between parts of the other two substituents. The effect was first reported by Beesley, Thorpe and Ingold in 1916 as part of a study of cyclization reactions.
A common application of this effect is addition of a quaternary carbon (e.g., a gem-dimethyl group) in an alkyl chain to increase the reaction rate and/or equilibrium constant of cyclization reactions. An example is this is an olefin metathesis reaction:
One proposed explanation for this effect is that the increased size of the substituents increases the angle between them. As a result, the angle between the other two substituents decreases. By moving them closer together, reactions between them are accelerated.